Add a Text Annotation Area for Multi-Page Document Annotation

## Description:
Hi, thank you for your great work on Label Studio!

In the current “Multi-Page Document Annotation” feature, we can annotate bounding boxes (bbox) on multiple images. However, I hope to further enhance the annotation workflow by adding a text annotation area to the interface.

## Feature request:

Add a text annotation area that displays the recognized question texts from all images.
The question texts can be prepared in advance by users and uploaded together with the images.
Annotators can add placeholder symbols in the question text corresponding to each bbox to indicate which part of the text the bbox belongs to.
This feature will help annotators more efficiently and accurately associate bbox annotations with the corresponding text content, especially in scenarios such as exam papers or structured documents.

## Example scenario:

Users recognize and extract all questions from the images in a multi-page document and upload the question texts together with the images.
In the annotation interface, these uploaded question texts are displayed in a dedicated text area.
Annotators can mark placeholder(s) in the question text to indicate which bbox is associated with which text.
Why is this important?
This feature can:

Greatly improve annotation efficiency and accuracy for structured documents.
Reduce ambiguity in bbox-to-text associations.
Additional context:
If you need further examples or UI sketches, I am happy to provide them.

Thank you for considering this request!